The Blood Gas Machine and Monitoring Gas Delivery in Anaesthetics Flashcards
What blood gas results are calculated?
- bicarbonate
- base excess
What blood gas results are measured?
- pH
- PCO2
- PO2
- electrolytes
- Hb
How is pH measured in the blood gas machine?
- using a pH electrode
- a potential difference develops across a pH sensitive glass bulb that is caused by, and proportional to, the difference in acid concentration on either side of it
- the 1st electrode is a mercury/mercuric chloride reference electrode with a semi-permeable membrane
- the 2nd electrode is silver/silver chloride sensing electrode which is kept at constant pH with a KCl buffer and encased in a pH sensitive glass bulb
How is PO2 measured in the blood gas machine?
- by the Clark (polarographic) electrode
- this consists of a platinum cathode and silver anode which form a circuit via KCl
- powered by 0.6V battery
- electrons form at the anode from the reaction with KCl
- electrons then react with O2 and water at the cathode to produce hydroxyl ions which generates a current
How is CO2 measured in the blood gas machine?
- by the severinghaus electrode
- uses the henderson hasselbalch equation
- there is a linear relationship between log PCO2 and pH
- so essentially a modified pH electrode that measures the pH change in an electrolyte solution when CO2 diffuses into it
- the pH sensitive glass is separated from the blood sample by a rubber/teflon membrane
- CO2 diffuses across the membrane to react with H2O on the other side, creating H+ ions -resulting pH change then measured
- slowed by diffusion of CO2 across membrane
How is the actual bicarbonate measured in the blood gas analyzer?
- using the measured pH and PCO2 of a sample in the Henderson Hasselbalch equation
- this gives the true plasma bicarb concentration
How is the standard bicarbonate calculated in the blood gas analyzer?
- plasma bicarbonate concentration after the sample has been corrected to a PCO2 of 5.3kPa at 37C
- this removes any resp component of acidosis/alkalosis - so only shows the metabolic component of any derangement in bicarb
- this can be derived from the Siggaard - Anderson nomogram
How are electrolytes measured in the blood gas machine?
Using specific ion-selective electrodes, which work on a similar principle to the pH electrode
How is Hb measured in the blood gas machine?
- using a co-oximeter
- this is a spectrophotometer that uses 4 different wavelengths of electromagnetic radiation to measure total Hb, oxyHb, carboxyHb and metHb and also gives the oxyHb saturation
- radiation is absorbed by substances with 2+ atoms
- by knowning the absorption characteristics of the substances to be measured, the appropriate wavelengths of radiation can be selected
- a photocell measures the amount of light it receives and this is compared to the reference photocell to quantify the amount of light absorbed
- from this amount the substance can be calculated
How is base excess calculated in the blood gas machine?
From the Siggaard - Anderson nomogram.
What is the base excess?
It’s the number of milimoles of acid required to titrate one litre of blood to pH 7.4, at a temperature of 37C with a PCO2 of 5.3 kPa.
What does the Siggaard Anderson nomograph measure?
The non-respiratory component of an acidosis/alkalosis.
It gives the quantity of acid/alkali required to turn the plasma in-vitro to a normal pH under standard conditions.
It examines the whole metabolic component of any acidosis/alkalosis.
What is standard base excess?
It’s the base excess value calculated for blood with an Hb of 5 g/dl (ie anaemic - because this is thought to better reflect the amount of acid/alkali required to correct any disturbance in-vivo since Hb is an effective plasma buffer but any acidotic/alkalotic process affects the whole body)
What is the Siggaard-Anderson nomogram?
- the PCO2 titration line is plotted for a blood sample by measuring the pH after the blood has equilibrated with 2 gas mixtures containing different concentrations of PCO2
- the gradient of the PCO2 titration line is determined by the buffering capacity of the blood
- therefore is related to the Hb concentration
- where the line meets the Hb curve - Hb of blood can be read
- the standard bicarb of blood is given by the intercept of the PCO2 titration line with the bicarbonate scale on the PCO2 = 40 mmHg line
- BE is given by the point at which the PCO2 titration line crosses the lower curve on the nomogram
What effect does excess heparin have on the ABG?
Heparin is acidic so it causes a spuriously low PCO2 and bicarbonate with a low pH.
What effect does a delay in analysis have on the ABG?
Continued metabolic activity of erythrocytes causes
- decreased pH
- decreased pO2
- increased pCO2
- secondary HCO3 decrease due to changes in pCO2
What effect does air bubbles have on the ABG?
- decrease pCO2
- increase pO2
- unless the pO2 of the sample is >21 kPa (ie more than air) then the bubbles cause a decreased PO2
What is the effect of a hypothermic patient on the ABG?
- decreased pH
- increased pCO2
- increased PO2
Because pH is a strong function of temperature, falling 0.015 units per degree celcius rise in temperature because the H+ ions dissociate.
The solubility of all gases decreases with increasing temperature - therefore a hypothermic patient will have a falsely elevated pCO2 and O2.
HCO3 will be decreased secondary to the changes in pCO2.
Does the solubility of all gases increase or decrease with increasing temperature?
The solubility of gases DECREASES with increasing temperature.
Why does pH fall with a rise in body temperature?
With inreasing temperature there is increasing dissociation of H+ ions
What can the Clark electrode monitor?
Only O2
What can a fuel cell measure?
Only O2
It works on the same principles as the Clark electrode but does not require a battery.
Anode is lead.
Cathode is gold.
Electrolyte solution is KOH. Electrons produced at anode and react with O2 at the cathode.
How does the paramagnetic analyzer work?
O2 is attracted toward a magnetic field (due to the unpaired electrons in it’s outer shell),
The analyzer measures the pressure differential between a stream of refernce gas (eg air) on one side and the sample gas on the other when exposed to an alternating magnetic field. A pressure transducer converts this pressure difference to an electrical signal calibrated to give the partial pressure of O2 on the sample.
How does mass spectrometry work?
- can measure any gas in a sample
- a few molecules of gas sample enter an ionization chamber where they are bombarded by electrons that move from a hot cathode to an anode
- this forms charged fragments of varying molecular weights
- these are accelerated onto the detector using a magnetic field or electrically charged rods
- by varying the current to the magnet/rods you can focus fragments of a particular size to land on the detector
- the amount of deflection depends on their mass
- the concentration can be measured from the output of the detector
How does gas chromatography work?
- uses a stationary phase (eg a column of tiny particles of silica-alumina coated in silicone oil) and a mobile phase (carrier such as helium)
- this separates the gas into it’s constituent components
- their separation dependent on their differential solubility in the 2 phases
- once separated the detector measures the concentration of components
How does Raman spectrometry measure gases?
- there is characteristic alteration in the freq and phase of scattered radiation as it passes through a specific transparent medium
- by passing a laser beam through a sample and processing the freq of the resultant scattered radiation the concentrations of a sample’s components can be estimated from the amplitude of the frequency shifted peaks
How law does the IR analyzer work on?
- infrared absorption occurs in proportion to agent concentration according to the Beer-Lambert law:
- A = log [Ii/It] = εLC
- A = IR Absorption
Ii = Incident intensity of IR beam
It = Transmitted intensity of IR beam
ε = the extinction coefficient
L = path length
C = Concentration of the absorbing gas
What do piezoelectric crystals measure?
Only gases/vapours that are soluble in oil, so only anaesthetic vapours.
They vibrate at specific resonant frequency when a current is applied across them. After coating them with oil, anaesthetic vapours in the gas sample dissolves into it, shifting their resonant frequency in proportion to the concentration of vapour present.
By measuring the frequency shift, the vapour concentration is calculated.
How does the refractometer work?
Not routinely used in anaesthesia gas analysis but identifies gases according to their different refractive indices.
Light split through a pair of chambers will produce characteristic interference patterns when recombined.
Two light beams are directed through different chambers, one with the gas/vapour in it.
The beams then combined and by analyzing the resulting dark and bright light patterns the concentration of a known gas can be measured.
